1. Field of the Invention
This invention relates in general to structural noise suppression systems particularly adapted to use in aircraft engine housings and, more specifically, to a double layer noise suppression honeycomb sandwich panel.
2. Description of the Prior Art
Since the earliest days of commercial jet aircraft, great efforts have been expended in developing methods and structures for reducing engine noise. Many different sound absorbing linings have been applied to intake bypass ducts, compressor casings and other components in aircraft turbine engine housings.
Early sound absorbing linings and panels used a honeycomb core bonded on one side to an impervious sheet and on the other to various pervious materials, such as perforated sheets, wire cloth or screens, fibrous layers and the like. Typical panels of this sort are described in U.S. Pat. Nos. 3,640,357 and 3,166,149.
In order to improve sound absorption, for some applications multilayer panels were designed, using plural honeycomb sheets separated by perforated sheets, with an impervious sheet on one side and a perforated sheet on the other, as described by Schindler in U.S. Pat. No. 3,948,346.
In further attempts to improve noise suppression, a variety of complex multi-component pervious sheets have been secured to cellular core structures. The previous sheets may include screens, fibrous mats, woven cloth, etc., as described by Cowan in U.S. Pat. No. 3,502,171. Multiple layers of honeycomb core material, perforated sheets and microporous sheets as described by Riel in U.S. Pat. No. 4,465,725 have been developed for specific applications.
U.S. Pat. No. 4,257,998 issued Mar. 24, 1981 teaches a method of making a cellular core with an internal septum. The septum is a perforated non-metallic sheet which in a softened state has the cellular core pressed into it to provide upper and lower core sheets having various predetermined heights.
Many of these designs are quite effective in reducing noise in aircraft engine applications. However, they tend to be complex in design and manufacture, expensive to manufacture, to add considerable undesirable weight to an aircraft and to have less than optimum strength and structural reliability when exposed to the hostile, high air flow environments encountered in engine housings.
Thus, there is a continuing need for effective noise suppression panels of improved simplicity, structural strength and light weight.